Lasers are used in a wide variety of systems, including laser marking systems that mark parts or products with markings or embossing. Such parts may include bolts, screws, product parts, or any other metal or nonmetal object, as understood in the art. To perform laser marking, lasers output high amounts of directed energy at particular wavelengths. Laser marking refers to a process of leaving marks on an object. Marking may include such methods as laser engraving, chemical/molecular alteration, charting, foaming, melting, and more. Laser marking can be applied in a number of industries, such as, but not limited to, metalworking, medical, automotive, electronics, wood, acrylic, leather, packaging, and more. Laser marking may be accomplished through a variety of technologies, such as FIBER, DPSS, and GAS lasers. One of ordinary skill in the art can appreciate that laser marking may be accomplished through other means.
For high-power lasers to be optimally used in commercial applications, there is a need to reduce signal structures typically observed in optical pulses emitted by high-power Q-switched all-fiber laser based on Yb-doped active fibers that cause laser output inefficiency. The optical pulses produced by this class of lasers are affected by amplitude modulation that can be detrimental for the overall laser efficiency. Strong amplitude modulations also introduce an unwanted variability in the optical output of different, yet nominally equivalent, laser systems. In fact, the characteristics of the modulation of the output is determined by a combination of different laser components, such as the active fiber absorption/emission properties, cavity length, cavity losses, and dynamics of the laser cavity Q-switching time.
All-fiber laser systems are often used in industrial applications with respect to their free-space counterpart because of a convenient alignment-free production process based on fusion splicing, robustness against environmental perturbations, and limited maintenance needs. However, cavity length of all-fiberized laser systems is typically longer than systems implementing free-space optical components. Therefore, laser cavities in the order of 4 m total length are typical for Q-switched lasers based on double-clad Yb-doped fibers pumped by state-of-the-art diode lasers emitting laser light in a 910-920 nm band. This laser cavity configuration is typically used in industrial laser systems for marking and machining applications. Cavity lengths of 4 m lead to photon round-trip times in the order of TRT=40 ns, and typical pulse lengths of 50 ns-200 ns at pulse repetition rates of 10 kHz-200 kHz.
The fingerprint of the cavity length is imprinted in laser output in the form of intensity modulation with frequency ωRT=2π/TRT. The amplitude of the modulation (i) depends on the cavity parameters, (ii) can be comparable to the pulse amplitude itself, and (iii) may lead to a complete splitting of the laser pulses under certain conditions. A strong modulation of laser output pulses has a direct impact on industrial applications, such as marking or engraving. While the total energy of the pulses is typically only slightly affected by this phenomenon, except when strong nonlinearities are triggered as further described herein, the pulse peak power may substantially change when a strong amplitude modulation is superimposed to the pulse envelope. If a maximum of the modulation corresponds to the center of the pulse, then the peak power is increased, otherwise the peak power is decreased. If the modulation is particularly strong, or if the laser is a operated close to the power-density threshold of non-linear effects, such as Raman scattering, stimulated Brillouin scattering, self-phase modulation, etc., a fluctuation of the peak power may substantially change the spectral characteristics of the laser output and reduce the overall efficiency of the laser.
To avoid the shortcomings of existing laser marking systems with regard to amplitude modulation being superimposed on laser pulses, conservative solutions are typically adopted. The most common strategy is to limit the total cavity gain by, for example, limited pumping, short active fiber absorption length, and high cavity losses (e.g., increase of the end-cavity transmissivity). These conservative methods reduce the amplitude of the pulse modulation, but also the total amount of the emitted energy-per-pulse.
In high power, compact systems, such as those used for laser marking, the conservative solutions described above have a negative impact on the final application. Hence, there is a need to solve the problem of inefficiencies of laser marking lasers due to amplitude modulation on laser pulses. Moreover, there is a need to reduce variability of output parameters of fiber lasers and reduce non-linear optical effects in fiber laser systems.